The present invention is directed towards new and entirely unexpected granular fertilizers and processes for making the fertilizers, which provide controlled release of the fertilizer to plants. The fertilizers may include additional advantageous agents such as micro and macro nutrients and other agriculturally beneficial materials. More particularly, the present invention is a granular fertilizer coated with a biologically degradable polymer (biopolymer). When such fertilizer is applied to the soil, microorganisms will degrade the biopolymer coating over time, releasing the fertilizer at a rate controlled by the amount, i.e., thickness of the biopolymer coating and chemical characteristics of the biopolymer, as well as environmental conditions.
There are many slow and extended release fertilizers with their nutrient release based on time and event related coating failures or coating permeability. These fertilizers are influenced by factors such as available soil water, fertilizer solubility, microbial activity in the soil, and the ratio of surface area to nutrient weight of the particle. Of these, the major commercial products are sulfur coated urea, polymer coated ureas, and urea-formaldehyde products such as methylene ureas.
Of the polymer coated fertilizers, almost all are dependent on ambient temperature and moisture, and the thickness of the coating. Most polymer coated fertilizers release nutrients by physical diffusion through a semi-permeable polymer membrane, and the release rate can be controlled by varying the composition and thickness of the coating. The type of fertilizer substrate also may influence the rate of nutrient (e.g., nitrogen) release. Nutrients are released by physical diffusion through the polymer coating.
A more recent polymer coated fertilizer employs a reactive layer coating which combines two reactive monomers as they are simultaneously applied to the fertilizer substrate. These reactions create an ultra-thin membrane coating, which controls nutrient release by osmotic diffusion. The coating thickness determines the diffusion rate and the duration of release for such reactive layer coated products.
Polymer-sulfur coated fertilizers are hybrid products that use a primary coating of sulfur and a secondary polymer coat. These fertilizers were developed to deliver controlled-release performance approaching that of solely polymer-coated fertilizers but at a reduced cost. The nutrient-release mechanism of these coated fertilizers is a combination of diffusion and capillary action. Water vapor must first physically diffuse in through the continuous polymer layer. The rate of diffusion is controlled by the composition and thickness of the polymer film. Once at the polymer-sulfur interface, the water penetrates the defects in the sulfur coat through physical capillary action and begins to dissolve the fertilizer core. The dissolved fertilizer then exits the particle in reverse sequence.
Thermoplastic resins such as polyolefins, polyvinylidene chloride, and copolymers may be employed to coat fertilizers. Because the thermoplastic polymers used are highly impermeable to water, release-controlling agents such as ethylene-vinyl acetate and surfactants are typically added to the coating to obtain the desired physical diffusion characteristics. Coating thicknesses are essentially the same for all of such fertilizers with the release being controlled by the chemical type of release-controlling agent and amount of the agent.